Submerged arc welding is enjoying a resurgence in popularity now that there is a greater awareness of safety considerations concerning fumes and arc flash. Direct current is most often used as the power source. The original D.C. automatic submerged arc welding applications utilized drooping power sources with voltage-following wire electrode feeders. It was not long before constant potential D.C. sources were introduced to the process, linked to constant speed wire electrode feeders.
Ferrous materials, most commonly steel, are magnetic, and because of this, D.C. welding is often not satisfactory. The magnetic field generated by the arc current and surrounding the arc and the fields associated with the ground currents react with each other in an unpredictable manner. The result is often referred to as "arc blow". This effect is most objectionable in deep groove welds where erratic movement of the arc disturbs proper formation and placement of the weld puddle.
When arc blow became a problem, users reverted to A.C. power, but common sine wave A.C. does not perform well in the submerged arc welding process. The submerged arc welding process typically runs at 24 to 38 volts. Using an A.C. trasformer type power source with these low voltages causes the arc to "stumble" and extinguish and then fail to re-ignite. This is because the sine wave A.C. exhibits a slow "crossover" where the voltage is a low or zero 120 times per second. It is during these times that the arc goes out, and then the wire must advance until it strikes the puddle again to begin re-ignition. This is not a good, stable arc condition.
An alternative way to use A.C. power has been with "drooper" supplies. These are typically 80 volts open circuit and designed with transformers of high leakage construction. They are basically constant-current sources with the running voltage established by the load, not by the power source. To use such a current source with the submerged arc process requires employment of a "voltage-following" wire feeder. The speed of this wire feeder is dependent upon the arc voltage, the feeder motor being driven by the secondary voltage. This type of wire feeder has not been widely used in A.C. welding because welding stability is difficult to obtain and hold. Current, voltage, and wire feed rate all interact with each other in an unpredictable manner, and the system is thus not stable and self-regulating. Moreover, starting current is very limited with drooper A.C. sources resulting in arc starts of marginal quality.
The present invention applies a square wave A.C. power source to the submerged arc welding process. The square wave has a very rapid crossover transition at polarity reversal, and thus the arc does not extinguish each half cycle. As a result, the wire does not need to advance, strike the workpiece, ignite a new arc and burn back 120 times per second.
Square wave arc welding power supplies are known in the prior art. One example is shown in U.S. Pat. No. 3,364,334 to Sato. This patent shows a diode bridge added in series with the A.C. line, and an inductor is connected across the D.C. terminals of the bridge. In this arrangement, the bridge is not the control element, and the welding current control must be provided on the A.C. line. In addition, the diodes of the bridge form a free wheeling path at all times.
In U.S. Pat. No. 4,038,515 to Risberg, the diodes and the bridge are replaced by thyristors. The actual welding current is compared with the desired weld current setting, and a firing control circuit responds to the resulting error signal to fire the thyristor bridge with proper phasing to supply a constant current to the load. This system cannot be used with a constant speed wire feeder. Therefore, the Risberg system could not be used in the submerged arc welding process unless it were tied to a variable speed wire feeder. The use of a variable speed voltage following wire feeder is not, however, a satisfactory solution because, as previously mentioned, cold starts are difficult, and any disturbance of the current or voltage would cause the voltage following wire feeder to react and interact with the arc in such a way as to become at least momentarily unstable.
What is needed is a constant current A.C. power source, controlled in a constant voltage mode, which can be matched with a constant speed wire electrode feeder, thus gaining the various advantages which have become familiar in D.C. automatic welding applications.